This may be an obvious answer when a new heater valve is cheap, but if you like to fiddle and have more time than money, then maybe it's worth a shot at repairing a heater valve.

If the valve leaks it is likely a lost cause, because the rubber diaphragm is likely fractured. If the valve is clogged or will not move, then it may be fixable. Being full of water stone (calcium, lime and rust) is common in warm climates where cars can run with plain water and no antifreeze (anti-corrosion agent).

It is possible to disassemble, clean, and reassemble the valve. The top is secured with a single pop rivet. Remove the rivet and rotate the top to unlock it from the body of the valve. There is a rubber diaphragm trapped between the top and bottom flanges. The operating mechanism is above the diaphragm, while the shut-off stopper is below the diaphragm. All of the hard crud will be underneath. With reassembly you have the get the top tight enough to seal around the edges of the diaphragm, and hope you don't split the rubber piece in the process.